1. Field of the Invention
The present invention relates to an ISFET, and in particular to a SnO2 ISFET device, manufacturing method, and method and apparatus to measure hysteresis width and drift rate therewith.
2. Description of the Related Art
ISFETs (Ion Sensitive Field Effect Transistor) are constructed by substituting a detecting film for the metal gate on the gate oxide of a traditional MOSFET (Metal-Oxide-Semiconductor Field Effect Transistor). When the ISFET is dipped into a solution, the interfacial potential between the detecting film and the solution influences the semiconductor surface since only an extremely thin dielectric (that is, the gate oxide) separates the detecting film from the semiconductor surface. This influences the charge density in the inversion layer of the semiconductor surface, and thereby modulates the channel current through the ISFET. Thus, by utilizing this characteristic, the pH or other ion concentration in a solution can be obtained from the measurement of source/drain current and the gate voltage of the ISFET. The potential difference on the interface between the detecting film and the solution is related to the ion activity in a solution. The hydrogen ion activity in the solution can be measured using different channel currents caused by different interfacial potential differences in various solutions with different hydrogen ion activity.
Patents related to the formation of the ISFET or measurement thereof are listed hereinafter.
U.S. Pat. No. 5,350,701 discloses a method of measuring the content of alkaline-group metals, especially the content of the calcium ions, utilizing chemosynthesis phosphide group as the detecting film on a gate of an ISFET.
U.S. Pat. No. 5,387,328 discloses a bio-sensor using ion sensitive field effect transistor with platinum, wherein an enzyme membrane is immobilized on the ion-detecting film to determine the concentration of glucose
U.S. Pat. No. 5,414,284 discloses a method of fabricating an ISFET and an ESD protective circuit on the same silicon substrate, wherein a capacitor is utilized as an interface between the protective circuit and a sample solution to the DC leakage current.
U.S. Pat. No. 5,309,085 integrates the measurement circuit of a creature sensor having ISFET on a wafer. The measured circuit has two ISFET devices, an enzyme ISFET and a reference electrode FET, whose output signal can be amplified by a differential amplifier.
U.S. Pat. No. 5,061,976 discloses a carbon thin film on the gate oxide of the ISFET and then a 2,6 xylenol electrolytic polymerization film formed thereon. The ISFET has the ability to detect hydrogen ions and the advantages of small floating time, high reliability, and insensitivity to light. When other film types are covered on the ISFET, other kinds of ions can be detected.
U.S. Pat. No. 5,833,824 discloses an ISFET sensor for detecting ion activity in a solution, which includes a substrate and an ISFET semiconductor die. The substrate has a front surface exposed to the solution, a back surface opposite to the front surface and an aperture extending therebetween. A detecting film of the ISFET is mounted on the back surface such that the gate region is exposed to the solution through the aperture.
U.S. Pat. No. 4,691,167 discloses a method of measuring ion activation in a solution by combining the ISFET, the reference electrode, the temperature sensor, amplifier circuit and a calculation and memory circuit. Since the sensitivity is a function of the temperature and drain current of ISFET and is decided by a variable of gate voltage, the sensitivity can be obtained by calculating formulas stored in memory.
U.S. Pat. No. 5,130,265 discloses a method of fabricating the ISFET with multiple functions. The method uses siloxanic prepolymer as the sensitive film, mixing the solution, photochemistry treatment and heat treatment.
U.S. Pat. No. 4,660,063 discloses a method of performing both laser drilling and solid diffusion to form a 3D diode array on the semiconductor wafer. The laser first drills the wafer, and the impurities are then diffused from the hole to form a cylindrical PN junction and complete a non-planar ISFET structure.
U.S. Pat. No. 4,812,220 discloses an ISFET made by fixing the enzyme on the detecting film to measure the concentration of amino acids in food. The enzyme sensor is miniaturized, and can accurately measure concentrations, even when small.
There are many materials acting as detection membranes of ISFETs, such as, Al2O3, Si3N4, Ta2O5, a-WO3, a-Si:H and the like. These thin films are deposited by either sputtering or plasma enhanced chemical vapor deposition (PECVD), therefore, the cost of the thin film fabrication is higher. For commercial purpose, it is important to develop a thin film, with low cost and easy fabrication.
In the ISFET applications, however, many factors such as hysteresis, temperature, and drift behavior affect the accuracy of the measuring results. Since pH-ISFET is a semiconductor device, it is easily influenced by variations in temperature. The variation of the temperature leads to a deviation of the measurement. With reference to the hysteresis behavior, it is related to the change in the pH of the solution (such as pHxxe2x86x92pHyxe2x86x92pHxxe2x86x92pHzxe2x86x92pHx) and the corresponding change in the output voltage of the ISFET (such as Vox1xe2x86x92Voyxe2x86x92Vox2xe2x86x92Vozxe2x86x92Vox3). At the same pH, the difference between the first output voltage and the final output voltage (such as Vox3xe2x88x92Vox1) is defined as the hysteresis width. For drift behavior, the drift rate is defined as the change in the gate voltage per unit time under conditions in which the source-drain current is stable and the temperature is constant after the intrinsic response of the pH-ISFET is completed. Hence, there is a need to measure the three effects to prevent error.
In view of this, an object of the invention is to provide a SnO2 gate ISFET. The invention forms the SnO2 layer as the detection membrane of the ISFET by sol-gel technology.
Another object of the invention is to provide a method of measuring temperature parameters of an ISFET. In the present invention, the sensitivities of the ISFET at different temperatures are obtained by the source-drain current and gate voltage of the ISFET in a solution, and temperature parameters (temperature coefficient of the sensitivity) of an ISFET are further obtained.
In the method of measuring the temperature parameters of an ISFET according to the present invention, the detecting film is immersed in a buffer solution, and, then, at a predetermined temperature, the pH of the buffer solution is changed to measure and record the source-drain current and the gate voltage of the ISFET to obtain a curve. The temperature parameters at the predetermined temperature are obtained by selecting a fixed current from the curve. The temperature parameters at other temperatures are obtained by changing the temperature of the buffer solution and the steps of measuring, recording and selecting.
Another object of the present invention is to provide a method of measuring the hysteresis width and drift rate of the SnO2 ISFET to use the reverse compensation method to obtain the accurate output value.
In the method of measuring the hysteresis width of a SnO2 ISFET according to the present invention, first, the drain-source current and then the drain-source voltage are fixed by a constant voltage/current circuit, and the SnO2 ISFET is immersed in a buffer solution. The gate-source output voltage of the SnO2 ISFET is recorded by a voltage-time recorder, and the pH of the buffer solution is changed. The steps of immersing and recording are then repeated to obtain the gate-source output voltages of the ISFET immersed in the buffer solution with different pH. The hysteresis width is the voltage deviation between starting pH and ending pH.
In the method of measuring the drift rate of a SnO2 ISFET according to the present invention, first, the drain-source current and then the drain-source voltage are fixed by a constant voltage/current circuit, and the SnO2 ISFET is immersed in a buffer solution. The gate/source output voltage of the SnO2 ISFET during a constant period is recorded by a voltage recorder. The pH of the buffer solution is changed and the steps of immersing and recording are repeated to obtain the gate-source output voltages of the ISFET immersed in the buffer solution with different pH. The drift rate is the slope of the gate-source output voltage with respect to time.
Another object of the present invention is to provide an apparatus to measure the hysteresis width and the drift rate. The apparatus of measuring the hysteresis width and the drift rate has a SnO2 ISFET, a buffer solution to contact the ISFET, a light-isolation container to load the buffer and to isolate light, a heater to heat the buffer solution, a constant current/voltage measuring device coupled to the source and drain of the SnO2 ISFET, and a voltage-time recorder to record the output voltage of ISFET.
A detailed description is given in the following embodiments with reference to the accompanying drawings.